Polymer dispersions derived from vinyl esters and monomers copolymerizable therewith, particularly vinyl acetate/ethylene (VAE) copolymers, and their use as binders for paints and plasters are well known. Such polymer dispersions are typically prepared by emulsion polymerization; with the polymerization being effected in one or more stages. In the polymerization processes which have become known to date, monomer mixtures are converted by free-radical polymerization by addition of initiators in the presence of protective colloids and/or emulsifiers. However, when used in binder-rich coatings (low pigment volume concentration or “pvc”), such as exterior wood protection paints, current VAE dispersions exhibit higher water uptake, higher algal and fungal growth, blistering and higher dirt pick-up than the pure acrylic emulsions normally used for exterior wood paints.
There is therefore a need to develop improved wood protection paints based on vinyl ester copolymers which are non-blistering and exhibit low water uptake, low algal and fungal growth, low dirt pick-up and high color retention.
U.S. Patent Application Publication No. 2009/0069495 to Fichtner et al. (“Fichtner”) is directed to coating compositions comprising binders based on polyvinyl ester dispersions prepared by a multistage polymerization process. In a first stage, a polyvinyl ester dispersion is prepared by polymerizing at least one vinyl ester of a saturated carboxylic acid (and optionally further monomers copolymerizable therewith) in the presence of at least one emulsifier and/or at least one protective colloid. In a second stage, monomers in a dissolved form, a pure form, or in the form of an emulsion are added to the reaction mixture and polymerized. The polyvinyl ester dispersions of Fichtner comprise polymerized silanes and/or epoxides, and are said to offer high weathering resistance when used in plasters and other exterior coatings, such as façcade paints. However, the dispersions of Fichtner suffer from the disadvantage of producing coatings with high water permeability (w-value).
U.S. Patent Application Publication No. 2009/0203814 to Petri et al. (“Petri”) discloses multistage polymers derived from hard and soft monomer compositions containing vinyl esters, of which at least one monomer composition must contain an unsaturated organosilicon compound. The dispersions of Petri are suitable for the formation of solvent-free coating materials which exhibit little foam formation and are said to offer improved blocking resistance, gloss and abrasion resistance in coatings produced therewith. However, the dispersions of Petri suffer from the disadvantage of a high dirt pick behavior when those dispersions are used in exterior paints and plasters.
U.S. Patent Application Publication No. 2007/0244238 to Desor et al. (“Desor”) is directed to polymer dispersions prepared by emulsion polymerization containing at least two copolymers, A and B, with glass transition temperatures differing by at least 10° C. In emulsion polymerization, usually one or more emulsifiers are used. Attempts have been made to avoid the use of alkylphenol ethoxylates (“APEO”) as nonionic emulsifiers due to health and environmental concerns. Accordingly, Desor's dispersions either contain no alkylphenol ethoxylates (“APEO”), or contain such small amounts thereof to be suitable for the preparation of wood varnishes, glazes and gloss paints. An APEO-free emulsifier is also provided by Desor, which ensures a desired shear stability of multistage polymer dispersions, and does not adversely affect the blooming and water resistance of the dispersion. However, the emulsions described in US 2007/0244238 display insufficient elasticity when used in exterior paint formulations for wood having pigments and more than 5% fillers.
U.S. Pat. No. 7,078,455 to Heldmann et al. (“Heldmann”) is directed to aqueous plastic material dispersions based on vinyl ester copolymers. More particularly, Heldmann's dispersion is based on a vinyl ester copolymer having a solid content of up to 80% by weight substantially stabilized by ionic components, and a minimum film formation temperature (MFFT) of less than 20° C. However, the dispersions of Heldmann suffer from the disadvantage of a high dirt pick behavior when the dispersions are used in exterior paints and plasters.
According to the present invention, it has now been found that a particular vinyl ester-ethylene acrylic dispersion prepared in a multistage polymerization process provides an exterior wood protection paint that is resistant to blistering and exhibits low water uptake, low algal and fungal growth, low dirt pick-up, high color retention and sufficient elasticity when applied on wood.